This invention relates to an apparatus and method for wetting powdered material, especially polymeric powdered material, so as to promote subsequent uniform distribution of the wetted material throughout bulk water.
It is well known that it can be difficult to dissolve water soluble powdered material quickly in water without forming lumps of aggregated partially dissolved material, often called fish-eyes.
Numerous mixing devices have been proposed with the aim of converting dry, substantially friable, particulate material into a uniform dispersion or solution in water.
A successful device is described in U.S. Pat. No. 4,086,663. This involves feeding particulate solids entrained in a stream of air centrally into a duct that is open at its lower end and that is provided with a plurality of sprays around the inlet for the entrained solids. The sprays are arranged to generate a spray of mist particles that substantially fill the duct. The individual polymer particles are wetted substantially individually while they pass down through the duct. They are collected in bulk water at the bottom of thE duct and, since they are individually wetted before they contact the water, they can dissolve or swell in the water rapidly and substantially independently of one another.
Using this system therefore it is necessary that the polymer fed into the duct is in the form of individual particles. Particulate solids still in a friable or loosely aggregated form must be broken into individual particles by entrainment in air before they enter the duct.
Another apparatus that preferably feeds the particulate solids into the device while entrained in air is U.S. Pat. No. 4,390,284. The solids may be fed into the device using a screw feeder. In this, a tangential stream of water is directed helically around a cone shaped inlet into which the entrained stream is directed, so as to wet that cone shaped inlet and prevent build-up of dry material on it. Wetting of the particulate material is substantially carried out by helically arranged jets of water in a cylindrical wetting duct. These jets are arranged to induce a considerable air flow through the cone shaped inlet and wetting duct.
A disadvantage of processes of this type is that they involve a large amount of air being fed into the top of the duct. Although the majority of the polymer particles are wetted while travelling through the duct and are collected in the bulk water beneath the duct, a large amount of air has to escape from the bottom of the duct above the bulk water and there is a risk that this air will carry away from the apparatus polymer particles entrained in the air. The risk is particularly significant for polymer particles that are finer than the majority of the particles for which the apparatus is designed. Accordingly the apparatus has a tendency to allow polymer fines to be blown away from the foot of the duct and this is undesirable, especially when the polymer powder contains a significant proportion of fines.
In U.S. Pat. No. 4,531,673 a polymer is fed by a screw feed into the top of a cone shaped duct and so it might be thought that this would reduce the risk of fines being blown out of the bottom of the duct. However the inlet arrangements at the top of the duct are designed to promote a flow of air being induced into and down the duct as a result of the downwardly directed water sprays. Since the apparatus is designed to induce this air flow, the problem of fines being blown out of the bottom of the duct still occurs.
In U.S. Pat. No. 4,643,582 the powdered material is fed into an upper conical inlet that leads into a cylindrical duct. A stream of water is directed tangentially into the conical inlet so as to flood its surface, and water is also directed tangentially into the cylindrical duct. Pressurised air is forced into an annulus around the conical portion and exits from this annulus into the cylindrical duct, thereby inducing suction into the centre of the conical portion so as to draw the powder into that. The pressurised air again incurs the risk of fines being blown out of the bottom. The use of a tangential water supply to flood the surfaces increases the risk of dry particles being partially dissolved in this excess water while in contact with one another, thus promoting the formation of fish-eyes.
It is an object of the invention to provide a simple apparatus and method for uniformly wetting water soluble or water swellable particulate material so as to permit that material to be dissolved or uniformly dispersed in bulk dilution water with minimum formation of fish-eyes, whilst maintaining a low level of build-up of solid material in the apparatus. It is a further object of the invention to achieve this with minimum escape of particulate fines from the apparatus during use. It is a preferred object of the invention to provide an apparatus and method for doing the above for water soluble or water swellable friable particulate material.